Figure 67. Waveguide dimensions.

2-7. SIZE

The correct size of a waveguide is

determined by the frequency (or wavelength) of the

energy that will be fed into the waveguide. Figure

67 shows that the narrow walls, or sidewalls, are

side a, and the top and bottom walls are side b. If

side b is one-half wavelength or less, cutoff will

occur. So the cutoff frequency can be determined

when b = /2.

Figure 66. Crisscrossing plane waves in a

waveguide.

a. To have energy travel through the

waveguide shown in figure 3-6 with minimum loss,

side b should be greater than one-half wavelength,

but less than 1 wavelength. The lower and upper

limits can be expressed, respectively, as /2 and . For the ideal waveguide the arbitrary figure for the width is b

= 0.7.

b. Side a of the waveguide is not critical since the field does not vary in this direction. However, the

dimension must be considered when determining the amount of power the waveguide must handle. Side a, the

smaller dimension, is where arc-over may take place. Also, if side a is greater than one-half wavelength, then a

half-cycle of vertical variation is possible and the signal will be attenuated. Therefore, side a should be smaller

than /2. The outside dimensions of a rectangular waveguide should be such that the width is twice the height.

The ratio of the inside dimensions is somewhat greater than 2 to1.

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